As is known, such devices are commonly used in machines for blow-moulding plastic containers in which a parison or a pair of parisons are housed in a mould where, by means of a combined stretching and blowing action, the container or containers is/are produced.
In more detail, said devices are used in rotary type blow-moulding machines, in which the moulds for producing containers are supported by a carousel which rotates about a vertical axis, between a parison loading station, in which a parison or a pair of parisons are inserted in the mould, and a container unloading station, in which the containers formed are picked up from the mould and transferred to subsequent stations.
The mould supporting carousel and, therefore, the moulds themselves, move with a continuous rotary motion, so the above-mentioned transfer devices must be used to perform two different operations: on one hand loading a parison or a pair of parisons in the mould, on the other hand picking up the containers formed from the mould.
To perform said operations, the pick up arm with the respective gripper must be able to perform combined translating and rotating movements so as to “follow” the rotary motion of the mould without interfering with it and at the same time must be able to load the parison or unload the container.
To achieve the above-mentioned combined movements, the supporting device comprises a cam equipped with a number of tracks which varies according to the number of degrees of freedom of the supporting arm.
The supporting arm in turn comprises cam follower wheels, each designed to engage with a respective track, for rotating and translating according to the trajectory which the grippers must follow.
As is known, each parison is picked up by the transfer device from a pre-heating oven and is then inserted in a respective mould by rotation of the transfer device.
In more detail, if the mould has a single cavity and therefore is shaped in such a way that it only houses a single parison, for producing a single container, the pick up arm has a single gripper designed to grasp a parison from the oven and to insert it in the mould. When the arm has a single gripper it must have at least two degrees of freedom to be able to correctly follow the moulds.
If the mould has two cavities and is therefore shaped in such a way that it can house two parisons, for producing two respective containers, each pick up arm is equipped with two grippers. When the arm has two grippers it must have three degrees of freedom to be able to correctly follow the moulds.
In prior art devices, every time it is necessary to pass from a single mould configuration to a double mould configuration the entire transfer device or at least the entire pick up arm guide cam must be substituted.
In particular, in the case of a single mould, the transfer device is equipped with a cam having two tracks (giving the above-mentioned two degrees of freedom of the gripper), whilst in the case of a double mould the transfer device is equipped with a cam having three tracks (giving the above-mentioned three degrees of freedom of the gripper).
Said cam substitution operations are very time consuming and extremely laborious, since a cam may weigh more than 50 kilograms.
Another disadvantage is at the parison loading station.
In the case of double moulds, the oven must also feed twice as many parisons per unit of time. Consequently, the transfer device must be able to pick up double the number of parisons without changing its speed of rotation, since the blow moulds continue to rotate at a constant speed (the presence of the double cavity does not mean that the speed of rotation of the moulds is increased).
For that reason, at the pick up station the grippers must have a tip speed greater than the case in which one parison at a time is fed.
To vary said tip speed, the pick up arm guide cam must again be substituted with the disadvantages described above.